With a high printing speed and energy saving of printing machines, there is an increasing demand for toners having an excellent low-temperature fusing property. However, when the toners are designed to have a low softening point or a low glass transition point in order to improve a low-temperature fixing property thereof, they tend to be deteriorated in heat-resistant storage property. Thus, the low-temperature fusing property and heat-resistant storage property of the toners are contradictory to each other.
In order to satisfy these contradictory properties, i.e., both the low-temperature fusing property and heat-resistant storage property of the toners, there have been proposed toners having a core-shell structure including a core portion and a shell portion formed of a resin different from that used for the core portion (for example, refer to Patent Document 1).
In addition, in order to further improve a low-temperature fusing property and a heat-resistant storage property of the toners, there have been developed technologies for producing the toners by using a hybrid resin having a chemical structure containing two kinds of molecular chains as a resin for the above core portion.
For example, Patent Document 2 discloses a toner of a core-shell type for development of electrostatic latent images in which a core layer contains core aggregates produced by aggregating core-forming particles containing a grafted polyester resin obtained by graft-polymerizing a polymerizable vinyl monomer with a polyester resin, a wax and a wax-dispersing aid for the purpose of improving a low-temperature fusing property and a storage stability of the toner having a core-shell structure.
Patent Document 3 discloses a toner for development of electrostatic latent images having a core-shell structure in which a shell is formed on a surface of respective core particles which include a resin binder containing a crystalline polyester resin and a non-crystalline polyester resin, and a colorant, and the core particles contain a composite resin formed by bonding a styrene-based resin and a polyester resin to each other for the purpose of improving a fixing latitude, a thermal storage property and a charging property of the resulting toner.
Patent Document 4 discloses resin particles of a core-shell type each including a shell layer formed of a resin (a) and a core layer formed of a resin (b) in which a weight ratio of the shell layer to the core layer is from 0.1:99.9 to 70:30, the resin (b) is formed of a polyester resin (b1) and/or a resin containing the polyester resin (b1) as a constitutional unit thereof, and further the resin (b1) is a polycondensed polyester resin obtained by subjecting a polyester to polycondensation reaction in the presence of a titanium-containing catalyst, for the purpose of improving a charging property, a heat-resistant storage property and thermal properties of the resulting toner.